Journal of The Academy of Clinical Microbiologists

: 2022  |  Volume : 24  |  Issue : 1  |  Page : 41--43

Aggregatibacter aphrophilus, a HACEK group pathogen as a cause of prosthetic valve endocarditis

Jyothi Embekkat Kaviyil1, Dinoop Korol Ponnambath1, Bijulal Sasidharan2, Pravi Vidyadharan1, Kavita Raja3,  
1 Department of Microbiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Medical College Campus, Thiruvananthapuram, Kerala, India
2 Department of Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Medical College Campus, Thiruvananthapuram, Kerala, India
3 Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Medical College Campus, Thiruvananthapuram, Kerala, India

Correspondence Address:
Kavita Raja
Department of Microbiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Medical College Campus, Thiruvananthapuram - 695 011, Kerala


Prosthetic valve endocarditis (PVE) is one of the most severe forms of infective endocarditis. Here, we describe a case of PVE caused by Aggregatibacter aphrophilus, a member of the HACEK group of pathogens, which is a difficult pathogen to identify using conventional methods. The pathogen was identified by 16s rRNA Sanger sequencing using eight-channel capillary electrophoresis method.

How to cite this article:
Kaviyil JE, Ponnambath DK, Sasidharan B, Vidyadharan P, Raja K. Aggregatibacter aphrophilus, a HACEK group pathogen as a cause of prosthetic valve endocarditis.J Acad Clin Microbiol 2022;24:41-43

How to cite this URL:
Kaviyil JE, Ponnambath DK, Sasidharan B, Vidyadharan P, Raja K. Aggregatibacter aphrophilus, a HACEK group pathogen as a cause of prosthetic valve endocarditis. J Acad Clin Microbiol [serial online] 2022 [cited 2023 Nov 30 ];24:41-43
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Infective endocarditis (IE) was first described by Sir William Osler in the mid-16th century, in his Gulstonian lectures to the Royal College of Physicians. It is an infrequent, but not rare disease often characterised by a high degree of morbidity and mortality.[1] Defined as an infection of the endocardial surfaces of the heart, it can cause severe valvular insufficiency, intractable congestive heart failure and myocardial abscesses.[1] Despite the major advances in every field of medicine, the mortality rates have not changed in the last quarter of a century.[2] Prosthetic valve endocarditis (PVE) accounts for 20% of all cases of endocarditis and occurs in about 6% of patients with prosthetic valves.[3] It is one of the most severe forms of IE and can be iatrogenic or community acquired, as in the case of late prosthetic valve infective endocarditis (PVE).[4] Viridans group of streptococci is considered to be the most common cause of IE in India, while Staphylococcus aureus is the most common cause of health care-associated IE. The major organisms causing Gram-negative endocarditis are the HACEK group and Pseudomonas aeruginosa that lead to native – as well as prosthetic valve endocarditis cases.[5] HACEK stands for Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens and Kingella species.

Here, we report a case of late prosthetic valve infective endocarditis caused by a HACEK group bacterium named A. aphrophilus.

 Case Report

A 49-year-old male presented with a fever of one month duration associated with intermittent chills and rigor and symptoms of heart failure. He had a history of rheumatic heart disease with surgical aortic valve replacement done 17 years before admission. Clinical examination showed a patient with features of severe aortic regurgitation in controlled heart failure. Laboratory investigations at the time of admission showed an elevated serum C-reactive protein (125.97 mg/l). Serological tests for HIV, HBV and HCV were negative. Echocardiography showed dehiscence of the prosthetic valve with severe paravalvar aortic valve regurgitation and mild left ventricular dysfunction. No vegetations were identified. New-onset prosthetic valve dehiscence was very suggestive of endocarditis. On the second day after admission, two sets of blood cultures were sent to the microbiology laboratory, one set each of aerobic and anaerobic cultures by BacT/ALERT system (bioMérieux, Inc., Durham, NC, USA). After four days of incubation, the aerobic blood cultures flagged positive and on Gram staining showed Gram-negative pleomorphic bacilli with slender forms that typically clustered together in the background of scattered RBC [Figure 1]. Subculture on blood agar did not yield any growth, however, on subsequent subculture with staphylococcal touch inoculation, colonies exhibiting satellitism were observed. On chocolate agar, the colonies were convex and translucent. Anaerobic blood culture did not yield any growth. On lawn culture with the X factor, V factor and XV factor discs on Mueller–Hinton agar, growth occurred around both V and XV factors. The isolate was sensitive to Ampicillin, Amoxicillin/Clavulanate, Cotrimoxazole, Ceftriaxone, Cefuroxime and Meropenem. For the identity of the pathogen, the culture was subjected to 16s rRNA sequencing by eight-channel capillary electrophoresis method (Sanger sequencing).{Figure 1}

Briefly, the 16s region of the rRNA gene was amplified using the universal primer sets.[6] The purified polymerase chain reaction (PCR) product was further used as the template for cycle PCR. Following PCR and post-sequencing clean-up of the amplicons, the denatured amplicons were loaded on to 3500 series genetic analyser (Applied Biosystems, USA) for capillary electrophoresis. The sequences obtained were analysed using the Bioedit tool and subjected to the Basic Local Alignment Search Tool (BLAST) for nucleotide similarity search. The genera and species were identified based on the per cent identity in BLAST. BLAST showed 98.87% identity with A. aphrophilus strain W10433 with 99% query coverage [Figure 2]. The strain was deposited at NCBI GenBank (Accession: OM278380.1, published January 20, 2022).{Figure 2}

The patient was treated with a combination of intravenous Ampicillin and Ceftriaxone, which were given for a total of 42 days. He became afebrile and the antibiotics course was completed and is awaiting surgery. Subsequent blood cultures done on samples taken about 40 days later did not show any growth.


According to a recent study by Arora et al. (2021), S. aureus amounts to the most common cause of IE, followed by viridans group of streptococci.[7] Although HACEK bacteria cause 5%–10% of native valve community-acquired endocarditis, they are rare causative agents of PVE amounting to only 1.4% of cases.[8] These are mainly commensals of the oral cavity that causes infections when an opportunity arises.[9] In this case, the patient had a history of dental infection which might have led to seeding of the bacteria into the circulation. A large multinational cohort study published in 2013 showed that Aggregatibacter amounts to 34% of all the HACEK IE cases, of which 6% was by A. aphrophilus.[10]

A recent article that did a registry-based comparative study confirmed that IE caused by the HACEK group has a better and more favourable prognosis when compared to the non-HACEK causes.[9] Earlier, Chambers et al. had noted that, 'the outcome of HACEK endocarditis was excellent overall with an in-hospital mortality of 3% which is less than one-quarter of the mortality for non-HE and one-sixth that of S. aureus endocarditis'.[10] Our patient was treated with IV Ampicillin and Ceftriaxone; third-generation cephalosporins are generally considered the treatment of choice.

A. aphrophilus is often found to cause culture-negative IE. This can pose difficulty in diagnosing IE caused by this organism. Although in our case, the aerobic blood cultures were positive, an attempt was made to confirm the identity of the isolated bacterium to standardise the procedure for the future. In suspected cases of culture-negative IE, target Sanger sequencing using the 16s rRNA and subsequent alignment search using an appropriate tool could prove to be an excellent diagnostic method to arrive at the aetiology. As shown by El-Dalati et al.,[11] the technique has potential to impact individual patient care by providing accurate aetiology. However, the disadvantage is the requirement for expensive machinery and associated consumables, and skilled workforce.


The authors would like to acknowledge the Molecular Genetics Unit of the Department of Biochemistry, SCTIMST for providing the facilities for carrying out the Sanger sequencing.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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